303372c41a8d5c58a46cf9ed595e30e67bd0bc99 docs: Improve netaddress comments (Carl Dong)

Pull request description:

Improves comments for `netaddress`, making them available to Doxygen.

I think this is worthwhile because a lot of the code require some context (e.g., A lot of the things that we do to fit hostnames and tor addresses into `CNetAddr` is non-obvious, and documenting it is beneficial).

ACKs for commit 303372:

Tree-SHA512: 2a35784a01ed8ec5fdbe111a540192d31bde16afa96e4be97b0385daf290fc7469a66d7cb8905a70b920fad6a0e7400ca4e5da082d6e4af1d1aaccc0e8297720

Backport of Core PR15718